Super D

Higher rotating mass over long distances may not be noticeable, but the effects are what they are. Think of it this way: if you're riding 80 miles, and before you leave, somebody straps a 1/4 lb weight to each foot, do you think that would have zero effect on climbs or acceleration or converting caloric expenditure into speed or distance? Probably not. I totally agree that you can't feel it, but physics is always in effect. You're lifting and then pushing a quarter pound additional weight around in a circle with each foot for 80 miles, or whatever the distance is. If it was pointless, there would be no need for bike racers to use lightweight shoes and bikes, sports and race cars wouldn't need light weight wheels and components to optimize performance, etc.

In terms of stiffness, the Diadora I tried on was noticeably softer flexing than my Shimano, and the Lake was incredibly stiff, much more so than the other two. Again, physics is a factor here. You're either going to transfer energy to the drivetrain, or you're going to absorb some of that energy with a flexible shoe. (Not to mention creating hot spots and other comfort/fatigue issues.) I've already tried riding with a semi-flexible shoe, so now I'm going to ride with a stiffer shoe and see what it's like. I'll still have to do some custom fitting on some level, no matter what the shoe, but that's normal.

I want to get a little closer and closer to efficiency with myself, my bike and my gear ongoing. So I'm including shoes in that equation.

In terms of getting what fits, you're spot on there. Has to fit well. If it fits poorly out of the box, most likely it's going to need either a lot of custom work (which ain't easy), or a ton of break-in (which means many painful miles), or it'll just completely suck the entire time until I give up and replace the shoes. Been down the completely suck road and I'm not doing that again, so I'm taking my time and getting a shoe that fits my foot shape, heel and has a toe box that doesn't hurt and rub my bony feet.